The present invention relates to a method and a system for automatically limiting the speed of a vehicle when freewheeling.
The present invention also relates to a computer program for executing such a method with a computer.
Automatic transmissions of the stage-geared gearbox type have become increasingly common in heavy-duty vehicles as microcomputer technology has continued to advance and has made it possible, with a control computer and a number of control elements, for example servo motors, to precision-control the engine speed, the connection and disconnection of an automated clutch between engine and gearbox and coupling members of the gearbox, relative to one another, so that smooth gearshift is always obtained at the correct rev speed. The advantage with this type of automatic gearbox compared with a traditional automatic gearbox constructed with planetary gear steps and with a hydrodynamic torque converter on the input side is firstly that, particularly as regards use in heavy vehicles, it is simpler and more robust and can be produced at substantially lower cost than the traditional automatic gearbox, and secondly that it has higher efficiency, which means the prospect of lower fuel consumption.
The automatic gearbox constructed from planetary gears usually has one-way couplings between the planetary gear steps, which, when the engine is in drive, lock in the automatic gear position for torque transmission from the engine to the drive wheels, but which, when torque is transmitted in the opposite direction, i.e. at zero throttle and with the vehicle in motion, disengage and cause the vehicle to freewheel without engine braking, which, by utilizing the kinetic energy of the vehicle, yields lower fuel consumption than if the engine is busy braking.
A corresponding freewheel function in automated stage-geared gearboxes is shown by Here, a freewheel function is obtained by putting a splitter gear disposed in the gearbox into its neutral position when a gas pedal disposed in the vehicle is located within a predetermined swivel angle range, which starts at a distance from the rest position of the pedal and extends over a slight angle within the total swivel angle of the pedal. When the driver wishes to change the driving condition from driving engine to allowing the vehicle to freewheel without engine braking, he lets up the gas pedal, so that it reaches the predetermined range when the drive from the engine is broken. If engine braking is required, he further releases the gas pedal, either fully or, in any event, to the point where it passes through the predetermined range within which disengagement occurs. With freewheeling technology according to W002/092378, the increase in speed of the vehicle when freewheeling can sometimes become excessive. In this case, the driver of the vehicle reduces the vehicle speed by activating the service brake and/or auxiliary brake of the vehicle. This is done manually and produces a somewhat jerky drive.
It is desirable to achieve an automatic freewheel function, in a vehicle with stage-geared automated gearbox, in which the risk of driving the vehicle at excessively high speed is reduced and a more comfortable drive is obtained, whilst, at the same time, the prospect is offered of improved fuel economy.
A method according to an aspect of the invention comprises a method for automatic freewheeling of a vehicle. In the vehicle are arranged speed-lowering means, at least one engine, at least one wheel driven by the engine via an automated stage-geared gearbox, a gas pedal for regulating the driving torque of the engine. The freewheeling is automatically activated in the presence of input signals indicating vehicle speed and gas pedal position within a predetermined swivel angle range of the gas pedal, which starts at a distance from the rest position of the gas pedal and extends over a slight angle within the total range of swivel of the gas pedal. The method comprises the steps whereby:                the activated freewheel function is deactivated when a predetermined vehicle speed limit is exceeded,        the vehicle is automatically braked with the speed-lowering means, whereupon        the freewheel function is reactivated once the vehicle speed has fallen to close to equal to the predetermined vehicle speed limit.        
An advantage associated with aspects of the method according to the invention is that the vehicle speed is automatically limited and, in connection therewith, that the freewheel function is automatically activated and deactivated according to requirement. The function according to aspects of the invention is integrated in the freewheel function. The driver enjoys a more relaxed and more comfortable drive. The fuel-saving benefit and other benefits of the freewheel function are enhanced by the present invention through reduced manual actuation of the freewheel function.
According to an aspect of the method according to the invention, the vehicle speed limit is predetermined in dependence on at least the prevailing road gradient of the road in which the vehicle is found. The vehicle speed limit can then be adjusted to different road gradients, so that the freewheel function and subsequent braking can be better realized.
According to a further aspect of the method according to the invention, an identification is made of the fact that the downhill slope on which the vehicle is traveling will end within a near future. This can be done with the aid, for example, of a GPS-based navigation system Positioning System) disposed in the vehicle. An alternative might be to predict the future topography of the vehicle with the aid of extrapolation. Once identification has been made that the downhill slope will end within a near future, the freewheel function is reactivated some time before the vehicle has fallen below the vehicle speed limit. The time is dependent, at least, on when the downhill slope is calculated to end.
This aspect of the invention can further extend the total freewheeling time and hence further reduce fuel consumption. By knowing in advance that the downhill slope will soon come to an end and that the vehicle will subsequently, without active braking, reduce its speed to below the predetermined speed limit, a reasonable breaching of the predetermined speed limit is allowed during the freewheeling. The freewheeling can therefore, in certain situations, continue to be activated in spite of the fact that the speed of the vehicle is somewhat higher than the predetermined speed limit.
A further aspect of the present invention also comprises a system for automatic freewheeling in a vehicle in which a control unit is arranged to, when the freewheel function is activated and in the presence of an input signal indicating that a predetermined vehicle speed limit is exceeded, to automatically deactivate the freewheel function and brake the vehicle with speed-lowering means, the control unit being arranged to reactivate the freewheel function in the presence of an input signal indicating that the vehicle speed has fallen to close to the predetermined vehicle speed limit.
The same advantages are obtained with aspects of the systems according to the invention as with aspects of the method according to the invention.
In one aspect of the system according to the invention, a system for setting the predetermined vehicle speed limit is coupled to the control unit. A vehicle speed can thereby be chosen, the breaching of which can initiate an automatic deactivation of the freewheeling. The device for setting the predetermined vehicle speed limit can be constituted by controls disposed on the vehicle instrument panel. According to a further embodiment of the system according to the invention, the vehicle speed limit can be determined in dependence on at least the prevailing road gradient of the road in which the vehicle is found. The vehicle speed limit can then be automatically adjusted by the device for setting the predetermined vehicle speed limit.
In a further aspect of the system according to the invention, the control unit is arranged to, during travel, at least on the basis of information on instantaneous vehicle speed, road gradient and gas pedal position, to continuously perform data simulations for the future progression of the vehicle with a view to optimizing the automatic activation and deactivation of the freewheeling with respect to a chosen criterion. A chosen criterion can be constituted by a demand for increased comfort and/or reduced fuel consumption. This embodiment allows the system according to the invention to take greater account of future topography in order better to be able to control the freewheel function. The activation and deactivation of the freewheel function with subsequent deceleration is thereby able to be further optimized.